I am unable to create new users on the main site of my

I am unable to create new users on the main site of my multi-site installation.

I am logged in as a super admin. When attempting to create the user I receive the following message.

Sorry, that email address is not allowed!

I created the user on one of the subsites/subdomains with success. I tried to look up the user, and the system does not find the username or email, but the system still will not let me create the new user.

I have tried multiple email addresses with no success.